Electron cyclotron resonance heating device



Maren 4, 155

TARO DODO ET AL ELECTRON CYCLOTRON RESONANCE HEATING DEVICE Filed Sept.27, 1965 INVENTOR. THRO DOD NHNHBlL YRMHMOTO alu mulem United StatesPatent US. Cl. 315-39 Int. Cl. H01j 7/46, 19/80, 7/24 1 Claim ABSTRACTOF THE DISCLOSURE An electron cyclotron resonance heating device of atype wherein a discharge plasma is generated in the magnetic field alongthe axial direction, into which a high frequency incidentelectromagnetic wave is projected perpendicularly with respect to thedirection of the magnetic field, thereby causing the electrons in theplasma to absorb the energy of the incident electromagnetic Wave, andthe electrons in the plasma are further heated to a high temperature soas to cause a light beam of short wavelength, in the far ultravioletregion, to emanate from the plasma.

This application is a continaution-in-part of prior application Ser. No.252,484 filed on Jan. 18, 1963, now abandoned, in the name of Taro Dodoand Manabu Yamamoto, and entitled Electron Cyclotron Resonance HeatingDevice.

This invention relates to a novel electron cyclotron resonance heatingdevice for generating far ultraviolet radiation of great importance andvalue for such work as spectroscopic analysis and research onphotochemical reactions.

As is known, the range of light wavelengths from several angstroms to2000 angstroms is generally referred to as the far ultraviolet region.Since, in such work as experimental spectroscopic analysis in thiswavelength region, vacuum spectrometers are required because ofabsorption due to air, this region is also known as the vacuumultraviolet region. Because the spectra of molecules, atoms, and ionshaving excitation energies of approximately 6 ev. or more fall Withinthis region, far ultraviolet spectroscopy is indispensable in the studyof the molecular structures and electronic states of such molecules,atoms, and ions. Furthermore, since high-temperature plasmas of severaltens of thousands of degrees Kelvin emit far ultraviolet light, farultraviolet spectroscopic technique is considered to be necessary formeasurement of high temperature plasmas. This technique is applied alsoto such observations as those of the sun, stars, and night sky and isuseful for attaining knowledge relating to space.

When this technique is further applied to spectroscopic analysis, sincethe intense spectrum lines of nonmetallic elements are Within the farultraviolet region, the technique is of great importance in the analysisof nonmetallic impurities in the refining processes of steel and othermetals. For example, the sensitive lines of such substances as carbon,phosphorus, and sulfur of relatively high composition content in ironand steels are within the far ultraviolet region; accordingly, qualitycontrol of even higher precision than that possible heretofore shouldbecome possible with progress in techniques in far ultravioletspectroscopic analysis.

In this far ultraviolet region, however, such ditficulties as loweringof transparency and reflection coefiicients of prisms and reflectingmirrors and the lowering of the detector sensitivity are encountered.For this reason, a light source of as high an intensity as available isrequired. Among the light sources used heretofore, many have been of thetype wherein capacitors charged to high voltages of approximately 10,000volts are discharged in a vacuum, examples of such sources being hotspark devices, sliding spark devices, and the Lyman tube. However, inall of these devices, reproducibility is poor, and the serviceable lifeis short because of erosion of such parts as electrodes and tube wall.Furthermore, since high voltage is used, these devices have furtherdisadvantages such as the occurrence of discharge between the dischargetube and the spectrometer or the contamination of the spectrometerinterior by the spatter of the electrodes and tube wall.

In addition, light sources in which, instead of capacitor discharge,continuous discharge in hydrogen gas or rare gases is utilized have alsobeen used. In the case of many of these light sources, however, thegreater part of the radiation is in the visible or ultraviolet region,and the far ultraviolet radiation is weak. The reason for this is thatthe temperature of the plasma within the discharge tube is low, andintense far ultraviolet rays are not radiated from a plasma of atemperature below 10,000 degrees Kelvin generated by ordinary discharge.The basic reason for this deficiency will now be considered in somedetail.

The distribution of atoms or ions in the states of exciting energy underthe condition of thermal equilibrium may be expressed by the so-calledBoltzmann factor exp. (-E/kT), where E is the excitation energy, k isthe Boltzmann constant, and T is temperature (degrees K.). When theabovesaid factor is calculated for excitation energy (approximately 6ev.) corresponding to a wavelength of 2000 angstroms, the result isapproximately 1/1000 for T=10,000 deg. K., approximately 1/30 forT=20,000 deg. K., and approximately 1/10 for T=30,000 deg. K. That is,by increasing the temperature three times, the in tensity of light of2000 angstrom wavelength is increased times. This ratio of intensityincrease becomes even greater with shorter Wavelengths. Accordingly, inorder to cause the radiation of considerably intense far ultravioletrays from light sources used at present, it is desirable that thetemperature be at least 20,000 degrees K. and preferably 30,000 degreesK. or higher.

An important point to be noted here is that, for radiation of farultraviolet rays, it is not necessary for all of the particles in theplasma to be equally at a temperature of 20,000 to 30,000 degrees K. orhigher. The reason for this is that excitation and ionization of gasatoms are accomplished principally by the collision of high velocityelectrons with these gas atoms, and, provided that the kinetic energy ofthe electrons can be increased, the aim of generating far ultravioletrays is attained. More explicitly, causing the mean kinetic energypossessed by the electrons to increase, that is, in equivalent effect,elevating the electron temperature and preventing unnecessary kineticenergy from being supplied to ions and other gas particles are thefundamental conditions for generating far ultraviolet radiation withhigh efiiciency.

In view of the foregoing considerations, it is an object of the presentinvention to provide a new electron cyclotron resonance heating devicefor generating a special high-temperature plasma which, with relativelylow power supply, accomplishes generation of the above-mentioned farultraviolet radiation.

It is a specific object of the invention to provide a device as statedabove wherein energy is supplied to only the electrons within theplasma, without heating the ions, and the plasma is heated withextremely high efficiency.

The foregoing objects have been achieved by the present invention, inwhich the phenomenon of absorption 8,900 {NU/sec.) (1) where e iselectron charge; and m is electron mass. The second important quantityis the gyration frequency produced when the individual electrons becomeencompassed about the magnetic fiux and undergo so-called cyclotrongyration. When denoted by f this quantity is expressed by the followingequation:

f 1r-; ;=2,800B(mc./see.)

where e represents electron charge; and m is electron mass.

If, into this plasma as described above, an electromagnetic wave offrequency f is projected, the electromagnetic wave will be intenselyabsorbed by the plasma at a certain frequency and only at thisfrequency. As is well known, this phenomenon, which has been determinedtheoretically and confirmed experimentally, is caused by the resonanceof the afore-mentioned three frequencies, namely, the characteristicfrequencies f and f of the electrons within the plasma and the frequencyf of the incident electromagnetic wave, and is commonly referred to aselectron cyclotron resonance absorption. The condition for theoccurrence of this resonance when the direction of propagation of theelectromagnetic wave is parallel to the magnetic field is that thefrequency f be approximately as follows:

f=fc

The said condition when the said direction and said magnetic field aremutually perpendicular is that the frequency f be approximately asfollows:

Since this resonance absorption is due to individual or collectivemotion of the electrons, and the ions of atoms do not participatedirectly in this resonance absorption, the process of energy transferfrom the electromagnetic wave to the electrons is accomplished withextremely high efficiency.

The invention will become more clearly understood by the followingdetailed description when read in conjunction with the accompanyingdrawing the single view of which illustrates one embodiment of anelectron cyclotron resonance heating device according to the presentinvention.

Referring to the drawing, the device comprises: a discharge tube made ofinsulating material such as glass, one end in the axial direction of thetube being connected to a vacuum spectrometer entrance slit 13 throughan anode electrode 3 having in its center a slit 5 and the other endbeing provided with a gas inlet 14 for discharge or ionization gas; acathode electrode 2 disposed in the said tube in alignment with theaxial line of the tube 1, a D-C current or low-frequency voltage beingimpressed between the said anode and cathode so as to generate dischargeplasma 4 in the axial direction of the tube 1 through a subsidiarydischarge such as an arc discharge, etc. being carried out by theelectrodes; a pair of coils 6, 7 which are wound about the tube 1 togenerate a magnetic field occurring in the axial direction of the saidtube; a magnetron 8 to serve as a high-frequency power source; arectangular waveguide 9; a shorting plunger 10; an isolator 11; and athree stub matchin section 12. The discharge tube 1 passes through therectangular waveguide 9 orthogonally, so that any micro-wave whichpropagates Within the said waveguide 9 advances vertically with respectto the strong magnetic field in the axial direction of the tube createdby the coils 6 and 7.

Now, the operation of this device will be described. The discharge tube1 contains an ionization gas at a low pressure of the order of about InHg. This ionization gas is exhausted from the slit 5 during operation ofthe device and the amount of gas decreased for the portion isreplenished from the gas inlet 14. The gas to be used for this purposecan be of any kind in principle, but an inert gas is usually employed.Within this discharge tube containing a sealed-in gas, there isconducted an electric discharge such as, for.example, a D-C aredischarge between the anode and the cathode, whereby a discharge plasma4 is generated. At this time, a strong D-C magnetic field is impressedby the coils 6 and 7 in the direction of the tube axis, i.e., in theflowing direction of the discharge current. Next, the magnetron 8 isactuated to supply a microwave which is propagated in the directionorthogonal to the above-mentioned magnetic field in the axial directionof the discharge tube by means of the rectangular waveguide 9. Thismagnetron can be operated either continuously or on repeated pulse. Theoutput of the magnetron can be about 20 kw. peak at the time of pulseoperation and about 20 w. in average.

An incident electromagnetic Wave is thus supplied from the magnetron 8to the square or rectangular wave guide 9. Since the discharge tube 1 isdisposed through the side walls of the wave guide 9, the electromagneticwave, that is propagated through the wave guide 9, is projected into theplasma 4 in a direction perpendicular to the magnetic field.Accordingly, the oscillation frequency at which the said electromagneticwave gives rise to resonance absorption within the plasma 4 isdetermined from the aforestated Equation 4.

As an example, when the magnetic density B is 3 kilogausses, thecyclotron oscillation frequency is given from Equation 2 as 28700(mc./sec.). Moreover, when the electron density within the plasma is 10(cm. the plasma oscillation frequency is given by Equation 1 as 22800(mc./sec.). Accordingly, the resonance frequency is given by Equation 4as That is, when a microwave of a frequency of 9100 (mc./ sec.) isdirected into the plasma 4, an intense, mutual interference is caused,the microwave power is absorbed by the electrons within the plasma.During this operation, it is possible that some power passes throughwithout being absorbed by the plasma. However, since this power isreflected by one end 10 of the wave guide 9 and is directed again intothe plasma, the quantity of ineffective power is extremely small.

It is preferred to use a magnetron for the high frequency power source8, and its operation may be either continuous or a repeated-pulseoperation.

In the device of this invention of the above-described construction, theelectrons within the plasma absorb microwave power, and it is possibleto maintain, constantly, electron temperatures of 20,000 to 30,000degrees K. or higher. From the plasma of such extremely hightemperature, light rays in the far ultraviolet region are led out to theoutside as a parallel-ray light beam through a passage opening 5provided in the center of the electrode 3.

It is to be observed from the foregoing description that the electroncyclotron resonance heating device according to the present invention,differing from a simple plasma generating device of direct-current orlow-frequency discharge type, is one in which a high-frequencyelectromagnetic wave is supplied into a plasma to cause the power of thesaid electromagnetic wave to be absorbed by the electrons within theplasma, that is, to cause so-called electron cyclotron resonanceabsorption to take place, and energy is supplied to only the electronswithin the plasma, whereby a plasma of extremely high temperature, whichhas heretofore been unattainable, is generated with extremely highefliciency. Accordingly, the present invention provides a light sourcefor far ultraviolet radiation requiring relatively low power and isparticularly applicable to spectroscopic analysis, photochemicalreaction, and research on such subjects as the energy levels of atoms.

Although the present invention has been described in conjunction with apreferred embodiment thereof, it is to be understood that modificationsas variations may be resorted to therein with-out departing from thespirit and scope of the invention, as those skilled in the art willreadily understand. Such modifications and variations are considered tobe within the purview and scope of the invention and appended claims.

What is claimed is:

1. An electron cyclotron resonance heating device which comprises, incombination:

a sealed horizontal discharge tube of insulating material containing agas at low pressure;

a pair of electrodes disposed at each end of said discharge tube tomaintain direct current discharge plasma;

an external coil wound about said tube generating an axial magneticfield in said tube;

a waveguide traversed substantially at right angles by said tube; and

a high-frequency power source connected to said waveguide to supply anelectromagnetic wave propagating therethrough and being projected intosaid plasma in a direction perpendicular to said magnetic field to bringabout resonance absorption by its oscillation frequency 1, saidfrequency f satisfying the following relationship US. Cl. X.R.

